Drag

Drag on Basic Shapes

EA203/2001
Prof. Burks
03 Oct. 2011
Francisco Jeldres N.
Abstract
This experiment wanted to show that the differences in drag are produced only due to different shapes and surface finish for the same cross-sectional area and same flow velocities for different models. As drag is proportional to dynamic pressure and cross-sectional area, the experiment consisted indifferent figures with the same cross-sectional area exposed to different flows velocities and with different surface finish or different shapes.

Introduction

In fluid dynamics, drag refers to forces that oppose the relative motion of an object through a fluid. The amount of drag experienced by an object is based on several factors including size, shape, surface finish, velocity, temperature,density and viscosity.
This experiment studied the relationship between drag, airspeed and different shapes and surface finish. The calibration for the USNA Flow Bench obtained in a previous experiment was used to know the flow’s velocity from the voltage.
The results were used to know the differences in drag between the objects, and associate this with shape and surface finish.
Drag isproportional to the dynamic pressure (q) and to the area (S) as shown:
D α q S (1)
The dynamic pressure is defined from the density (ρ) and the velocity (V) of the freestream air as follows:
q = (1/2) ρ V2 (2)
Density was calculated from the equation of state:P = ρ R T (3)
Equation (1) is rewritten as follows:
D = Cd q S (4)
Where Cd is called Coefficient of Drag and it was due to objects shape and surface finish, orientation to flow, Mach number, and Reynolds number, which was calculated from the density (ρ),velocity (V), length (L, measured diameter), and viscosity (µ, determined based on temperature and the graph in page 232 of Anderson):
Re = ρ V L µ-1 (5)
Four different objects were tested in the experiment: a flat plate, a half sphere, a smooth sphere, and a tripped sphere which had a rough section on the side that faced the freestreamair. Every one of them had the same reference area (S), due to the fact that all had the same diameter. Therefore any difference in drag was due to shape and surface finish.
Methodology

Experimental set-up
The experiment was conducted in a closed room with air conditioning system. A barometer and a thermometer were used to measure pressure and temperature in the room, respectively.
Aroom’s blower was used to produce an upward flow. The flow’s intensity was controlled with a voltage controller, which had a read-out screen in volts.
A sensor to measure force was used to measure drag to the different shapes.
Procedure
The room’s temperature and the day’s barometric pressure were recorded based on the thermometer and the barometer, respectively.
The results of the airspeedexperiment were used to calculate the velocity to voltage conversion for the required test voltages. It started in 1.0 volts and finished in 5.0 volts with an increase of 0.5 volts in every step.
The diameter of all figures was measured and every one of them was installed onto the force balance at a height of eight inches above the opening of the flow bench.
The force balance was set in zero and thevoltage controller was turned on.
The drag was recorded for every voltage and for every model.
This permitted to obtain nine data points for every model.

Results
Raw Data
Drag was recorded for four different figures from 1.0 volts until 5.0 volts with an increment of 0.5 volts in every step. Table 1 shows the results for the force balance and the different shapes. Voltage is shown in...